Titanium dioxide coated with silicon dioxide

ABSTRACT

A powder containing particles with a core of titanium dioxide and a coating of silicon dioxide in an amount of between 0.5 and 40 wt. %, the particles have a BET surface of between 5 and 300 m 2 /g, and the particles are primary particles that have a coating of silicon dioxide and a core of titanium dioxide.

The present invention provides titanium dioxide powder, which is coveredwith a silicon dioxide coating and produced by flame hydrolysis, adispersion containing this powder, as well as the processes for theproduction of the powder and dispersion. The invention also provides forthe use of the powder and dispersion.

Metal oxides such as titanium dioxide or zinc oxide are widely used insunscreen agents. Their action basically involves reflection, scatteringand absorption of the harmful UV radiation and depends substantially onthe primary particle size of the metal oxides.

The disadvantage with metal oxides such as titanium dioxide or zincoxide is their photocatalytic activity, by means of which reactions aretriggered that can lead to changes in the constituents of a sunscreenagent.

Attempts have been made to reduce the photocatalytic activity of thesemetal oxides without reducing the UV-screening properties, by forexample covering them with a coating.

It is known to coat titanium dioxide powder with silicon dioxide. As arule titanium dioxide particles are dispersed in a liquid medium, thecoating component is added in the form of a silicate, and silicic acidis precipitated on the surface of the titanium dioxide particles. Thismay then be followed by heat treatment procedures. Instead of silicates,organosilicon compounds may also be used.

EP-A-0988 853 describes titanium dioxide particles coated with silicondioxide, as well as their production and use as a constituent insunscreen agents.

The disadvantage with these particles is their low surface functionalityand the high degree of intergrowth of the particles. On the one handthis complicates the incorporation of the particles in a cosmeticformulation, and on the other hand restricts their stability as regardssedimentation.

These disadvantages are largely overcome in the European patentapplication having the Application No. 01119108.7 of Aug. 8, 2002. Thetitanium dioxide particles coated with silicon dioxide that aredescribed there can easily be incorporated in cosmetic formulations, andare stable in the latter and have a low photocatalytic activity.

The disadvantage with the processes in which the coating is applied froma liquid phase remains however the reproducibility. Titanium dioxidepowders tend to form aggregates in the liquid dispersion phase. Thetitanium dioxide produced by flame hydrolysis that is preferably used inaddition exhibits an aggregated structure. This means that the coatingin each case surrounds aggregates and/or agglomerates. The structure ofthe aggregates and agglomerates is however strongly dependent on theconditions during the reaction, for example the pH value or dispersionenergy. It is therefore difficult to obtain a uniform powder in areproducible manner.

In addition to the processes in which the coating is applied from aliquid medium, processes also exist in which titanium dioxide particlescoated with silicon dioxide are produced in gaseous phase reactions.

Hung and Katz (U.S. Pat. No. 5,268,337) and J. Mater. Res. 7, 1861(1992)) describe the production of titanium dioxide particles coatedwith silicon dioxide in a flame hydrolysis process. For this purpose aburner is used, in which the precursors of the titanium dioxide andsilicon dioxide together with the combustible gas and an inert gas asstream 1 and an oxidising gas as stream 2 are fed in countercurrent anda diffusion flame is ignited at the point of impact of the two streams.If titanium tetrachloride and silicon tetrachloride are used asprecursors in a ratio of SiCl₄ to TiCl₄ of 1:1 to 3:1 and the flametemperature lies between 500° and 2300° K., the formation of a completesilicon dioxide coating takes place. The titanium dioxide core is inthis connection present in the rutile configuration. If the proportionof silicon dioxide is less, then simply the formation of silicon dioxidedomains on the titanium dioxide surface occurs.

The disadvantage with this process is that the burner with acountercurrent diffusion flame can be adapted only with difficulty tolarger, economically operating burners. A further disadvantage is thelarger proportion of silicon dioxide precursors that is necessary inorder to obtain a fully formed coating.

A further possible way of producing titanium dioxide particles coatedwith silicon dioxide is the gaseous phase oxidation, described in WO96/36441, of a thermally decomposable titanium dioxide precursor and athermally decomposable silicon dioxide precursor with oxygen in atubular reactor at temperatures of at least 1300° C. In this, theretakes place first of all the oxidation of the titanium dioxide precursorwith the formation of titanium dioxide particles, and only then is thesilicon dioxide precursor added to the reaction mixture.

The disadvantage of this process is that the use of a tubular reactorleads to non-uniform products. This is due to the fact that theresidence time of titanium dioxide particles formed by hydrolysis of thetitanium dioxide precursor is not uniform in the tubular reactor, andtherefore results in titanium dioxide cores of varying structure.

These in turn are not uniformly coated with silicon dioxide on accountof the non-uniform residence time over the reactor cross-section. Theresult is a non-uniform product as regards the titanium dioxide core andthe silicon dioxide coating. In addition there is also the fact thatunder the reaction conditions adhesions of undefined products on thereactor wall that can be incorporated into the product have to bereckoned with. The process described in WO 96/36441 accordingly does notprove to be an advantageous way of obtaining a uniform product.

The object of the present invention is to provide a titanium dioxidepowder coated with silicon dioxide that does not have the disadvantagesof the prior art. In particular, it should have a complete silicondioxide coating of largely uniform thickness, a low photocatalyticactivity, and should be able to be produced in a reproducible manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a TEM image of the powder from Example 1.

FIG. 2 shows the zeta potential curves of the powders according to theinvention of Examples 1 to 3 compared to titanium dioxide (P25 TiO₂ fromDegussa).

The invention accordingly provides a powder consisting of particles witha core of titanium dioxide and a coating of silicon dioxide, which ischaracterised in that it

-   contains a proportion of silicon dioxide between 0.5 and 40 wt. %,-   has a BET surface between 5 and 300 m²/g, and-   consists of primary particles that have a coating of silicon dioxide    and a core of titanium dioxide.

The proportion of silicon dioxide in the powder according to theinvention is between 0.5 and 40 wt. %. With values below 0.5 wt. % itcannot be ensured that a completely closed silicon dioxide coating ispresent. With values above 40 wt. % the titanium dioxide powders coatedwith silicon dioxide tend to have too low a UV absorption. The BETsurface of the powder according to the invention is determined inaccordance with DIN 66131.

Primary particles are understood to denote very small particles thatcannot be decomposed further without rupturing chemical bonds.

These primary particles may grow together to form aggregates. Aggregatesare characterised by the fact that their surface is less than the sum ofthe surfaces of the primary particles of which they are composed.Furthermore aggregates do not decompose completely into primaryparticles on dispersion. Powders according to the invention having a lowBET surface may be present wholly or largely in the form ofnon-aggregated primary particles, while powders according to theinvention with a high BET surface may have a relatively high degree ofaggregation or may be present in a completely aggregated form.

The aggregates preferably consist of primary particles that growtogether through their silicon dioxide coating. Powders according to theinvention based on such an aggregate structure exhibit a particularlylow photoactivity with a high absorption.

In addition the powder according to the invention may preferably have asilicon dioxide content of 1 to 20 wt. %.

The ratio of the rutile/anatase modifications of the titanium dioxidecore of the powder according to the invention may be varied within widelimits. Thus, the ratio of the rutile/anatase modifications may be 1:99to 99:1, preferably 10:90 to 90:10. The titanium dioxide modificationsexhibit different degrees of photoactivity. On account of the broadlimits of the ratio of the rutile/anatase modifications, together withthe silicon dioxide content of the coating, powders may specifically beselected for example for use in sunscreen agents.

The powder according to the invention may preferably have an absorptionat 320 nm of at least 95%, particularly preferably of at least 97%, andat 360 nm may preferably have an absorption of at least 90%,particularly preferably at least 92%. The absorption is in each casedetermined in an aqueous dispersion of the powder with a solids contentof 3 wt. %.

The powder according to the invention may preferably have aphotoactivity index of less than 0.5, particularly preferably less than0.3.

In the determination of the photoactivity index the sample to bemeasured is suspended in 2-propanol and irradiated for 1 hour with UVlight. The concentration of acetone that is formed is then measured.

The photoactivity index is the quotient of the acetone concentrationdetermined when using a powder and the acetone concentration determinedwhen using titanium dioxide P25, a pyrogenically produced titaniumdioxide from Degussa.

The acetone concentration in mg/kg may be used as a measure of thephotocatalytic activity of the sample since the formation of acetone canbe described by a reaction of zero order kinetics according to theequation dc[Ac]/dt=k.

The isoelectric point (IEP) of the powder according to the invention maypreferably lie at a pH value between 1 and 4, particularly preferablybetween 2 and 3.

Stable dispersions may thereby be produced for example in the rangebetween pH 5 and 7 of interest for sunscreen agents. Titanium dioxideparticles without a coating lead to unstable dispersions in this pHrange unless further additives are added to the dispersion.

The IEP indicates the pH value at which the zeta potential is zero. TheIEP in the case of titanium dioxide is at a pH of ca. 5 to 6, and in thecase of silicon dioxide is at a pH of ca 2 to 4. In dispersions in whichthe particles carry acidic or basic groups on the surface, the chargecan be altered by adjusting the pH value. The greater the differencebetween the pH value and the IEP, the more stable the dispersion.

The zeta potential is a measurement of the surface charge of particles.The zeta potential denotes the potential at the shear plane within theelectrochemical double layer consisting of particles of the powderaccording to the invention and electrolyte in a dispersion. The zetapotential depends inter alia on the nature of the particle, for examplesilicon dioxide, titanium dioxide or titanium dioxide coated withsilicon dioxide. Particles of the same material have the same sign ofthe surface charges and thus repel one another. If the zeta potential istoo small, the repulsive force may however not be sufficient tocompensate the van der Waals attraction of the particles, resulting inflocculation and possibly sedimentation of the particles.

The zeta potential of the powder according to the invention isdetermined in an aqueous dispersion.

Moreover the powder according to the invention preferably has a BETsurface of 40 to 120 m²/g, particularly preferably between 60 and 70m²/g.

The invention also provides a process for the production of the powderaccording to the invention, which is characterised in that a vaporisablesilicon compound and a vaporisable titanium compound are mixed inamounts corresponding to the subsequently desired ratio of SiO₂ and TiO₂in the product, are vaporised at temperatures of 200° C. or less, andare transferred by means of an inert gas stream together with hydrogenand air or with oxygen-enriched air to the central pipe (core) of aknown burner, the reaction mixture is ignited at the mouth of the burnerand is introduced together with secondary air, is combusted in a cooledflame pipe, following which the titanium dioxide powder coated withsilicon dioxide is separated from the gaseous reaction products and ifnecessary is freed in moist air from adhering hydrogen chloride, whereinthe ratio of

-   -   primary air to secondary air is greater than 0.3,    -   core hydrogen to secondary air is greater than 1,    -   titanium dioxide precursor to secondary air is greater than 0.5

It has been found that the powder according to the invention is obtainedonly if all the specified parameters are observed. If there arefluctuations, then powders and powder mixtures not in accordance withthe invention are obtained. Thus, for example, without the addition ofsecondary air and without observing the ratios of secondary air toprimary air/core hydrogen/titanium dioxide precursor, silicon/titaniummixed oxide powders are obtained in which silicon and titanium arehomogeneously distributed. Such a mixed oxide powder is described inDE-A-4235996.

The type of vaporisable titanium compound in the production of thepowder according to the invention is not restricted. Titaniumtetrachloride may preferably be used.

The type of vaporisable silicon compound is likewise not restricted.Silicon tetrachloride may preferably be used.

The invention also provides sunscreen agents that contain-the powderaccording to the invention in an amount of 0.01 to 25 wt. %. In additionthe sunscreen agent according to the invention may be used in mixtureswith known inorganic, UV-absorbing pigments and/or chemical UV filters.

Suitable known UV-absorbing pigments include titanium dioxides, zincoxide, aluminium oxides, iron oxides, cerium oxides, zirconium oxides,barium sulfate or mixtures thereof.

As chemical UV filters there may be used all water-soluble oroil-soluble UV-A and UV-B filters known to the person skilled in theart. Selected examples that may be mentioned include:2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxybenzophenone-5-sulfonic acid,2-hydroxy-4-methoxybenzophenone-5-sulfonate sodium salt,dihydroxydimethoxybenzophenone, dihydroxydimethoxybenzophenonesulfonatesodium salt, tetrahydroxybenzophenone, p-aminobenzoic acid,ethyl-p-aminobenzoate, glyceryl-p-aminobenzoate,amyl-p-dimethylaminobenzoate, octyl-p-dimethylaminobenzoate,ethyl-p-methoxycinnamate, isopropyl-p-methoxycinnamic acid ester,octyl-p-methoxycinnamic acid ester, 2-ethylhexyl-p-methoxycinnamic acidester, p-methoxycinnamic acid ester sodium salt,glyceryl-di-p-methoxycinnamic acid ester, mono-2-ethylhexanoate, octylsalicylate, phenyl salicylate, homomenthyl salicylate, dipropyleneglycol salicylate, ethylene glycol salicylate, myristyl salicylate,methyl salicylate, 4-tert.-butyl-4-methoxydibenzoylmethane and2-(2′-hydroxy-5′-methylphenyl)-benzotriazole. Of these,2-ethylhexyl-p-methoxycinnamic acid ester and4-tert.-butyl-4′-methoxydibenzoylmethane are preferred on account oftheir UV protection and the fact that they are harmless to skin.

The sunscreen agents according to the invention may furthermore containthe solvents known to the person skilled in the art, such as water,monohydric or polyhydric alcohols, cosmetic oils, emulsifiers,stabilisers, consistency regulators such as carbomeres, cellulosederivatives, xantham gum, waxes, bentones, pyrogenic silicic acids andfurther substances conventionally used in cosmetics, such as vitamins,antioxidants, preservatives, colourants and perfumes.

Typically the sunscreen agent according to the invention may be presentas an emulsion (O/W, W/O or multiple), an aqueous or aqueous-alcoholicgel or oil gel, and may be presented in the form of lotions, cremes,milk sprays, mousse, as a stick, or in other conventional forms.

The general structure and formulation of sunscreen agents is furthermoredescribed in A. Domsch, “Die kosmetischen Präparate”, Verlag fürchemische Industrie (Editor: H. Ziolkowsky), 4^(th) Edition, 1992 or N.J. Lowe and N. A. Shaat, Sunscreens, Development, Evaluation andRegulatory Aspects, Marcel Dekker Inc., 1990.

The invention in addition provides for the use of the powder accordingto the invention as a UV filter, and for the production of dispersionsand use for chemical-mechanical polishing (CMP process).

The process according to the invention permits the production oftitanium dioxide particles completely coated with silicon dioxide,wherein only minor amounts of silicon dioxide precursors are necessary.With known processes in which the production is carried outpyrogenically, substantially larger amounts of silicon dioxideprecursors are necessary in order to achieve a complete coating. This inturn reduces the UV absorption of the powders according to the priorart.

The unique structure of the powder according to the invention, in whichthe primary particles already have a sealed coating, also means that, inthe event of a possible aggregation of the primary particles, noaggregation of the titanium dioxide cores is possible. With the knownpowders, which are obtained by coating a titanium dioxide core in anaqueous medium, as a rule there is an aggregation of the initialtitanium dioxide powders, or aggregated powders are used to start with.In these processes aggregates are thus coated, and not primary particlesas in the case of the process according to the invention.

On the basis of the facts known from DE-A-4235996 it was not expectedthat a completely different powder would be formed by addition ofsecondary air under specifically defined ratios to the rest of the gasesthat are used. Whereas the process described in DE-A-4235996 leads to amixed oxide powder with an homogeneous distribution of titanium dioxideand silicon dioxide, the process according to the invention produces apowder with a complete coating of silicon dioxide and a core of titaniumdioxide.

EXAMPLES Analytical Measurements

The titanium dioxide and silicon dioxide contents are determined bymeans of X-ray fluorescence analysis.

The BET surface is determined according to DIN 66131.

The dibutyl phthalate absorption (DBP number) is measured using aRHEOCORD 90 instrument from Haake, Karlsruhe. For this purpose 16 g ofthe silicon dioxide powder are weighed out to an accuracy of 0.001 g andadded to a kneading chamber, which is closed with a lid and into whichdibutyl phthalate is metered in at a predetermined metering rate of0.0667 ml/sec through a hole in the lid. The kneader is operated at amotor speed of 125 rpm. After the maximum torque is reached the kneaderand the DBP metering device are automatically switched off. The DBPabsorption is calculated as follows from the consumed amount of DBP andthe weighed-out amount of the particles:DBP number (g/100 g)=(consumption of DBP in g/weighed-out amount ofpowder in g)×100.

The pH value is determined in accordance with DIN ISO 787/IX, ASTM D1280, JIS K 5101/24.

Determination of the photoactivity index: ca 250 mg (accuracy 0.1 mg) ofthe particles obtained from the examples and comparison examples aresuspended in 350 ml (275.1 g) of 2-propanol using an ultra-turraxstirrer. This suspension is conveyed by means of a pump through a coolerthermostatically controlled to 24° C. into a glass photoreactorpreviously flushed with oxygen.

An Hg medium pressure TQ718 type immersion lamp (Heraeus) with an outputof 500 watt is used for example as radiation source. A protective tubeof boron silicate glass restricts the emitted radiation towavelengths >300 nm. The radiation source is surrounded externally by acooling pipe through which water flows.

Oxygen is metered into the reactor through a flow meter. The reaction isstarted when the radiation source is switched on. At the end of thereaction a small amount of the suspension is immediately removed,filtered and analysed by means of gas chromatography.

UV-visible wavelength spectra (absorption) are measured in 3 wt. %dispersions using a Specord 200 UV-visible range spectrophotometer witha photometer sphere (Analytikjena AG).

The zeta potential and the isoelectric point are determined according tothe DVI method in a 10% aqueous dispersion of the powders according tothe invention using a DT-1200 type instrument from Dispersion TechnologyInc.

Example 1

3.86 kg/hour of TiCl₄ and 0.332 kg/hour of SiCl₄ are vaporised at ca.200° C. in a vaporiser. The vapours are mixed by means of nitrogentogether with 1.45 Nm³/hour of hydrogen and 7.8 Nm³/hour of dried air inthe mixing chamber of a burner of known design and construction and fed,through a central tube at whose end the reaction mixture is ignited, toa water-cooled flame tube, where they are combusted. In addition 0.9Nm³/hour of hydrogen and 25 Nm³/hour of air are fed to the flame tubethrough a jacket tube concentrically surrounding the central tube.

The powder that is formed is then separated in a filter. Adheringchloride is removed by treating the powder with moist air at ca.500-700° C. The powder contains 92 wt. % of titanium dioxide and 8 wt. %of silicon dioxide.

The Examples 2 to 5 are carried out similarly to Example 1. The batchsizes and the experimental conditions are given in Table 1, and thephysicochemical properties of the powders according to the invention aregiven in Table 2.

TEM-EDX evaluations of the powders of Examples 1 to 5 show a largelyaggregated powder with a complete silicon dioxide coating and a titaniumdioxide core. FIG. 1 shows a TEM image of the powder from Example 1.Aggregates are present in the powder, the primary particles formingintergrowths via the silicon dioxide coating. The BET surface is 66m²/g. X-ray diffraction analysis shows a rutile/anatase ratio in thecore of 26:74.

FIG. 2 shows the zeta potential curves of the powders according to theinvention of Examples 1 to 3 compared to titanium dioxide (P25 TiO₂ fromDegussa). It can be seen that, with increasing content of silicondioxide, the isoelectric point is displaced towards lower pH values andeven at the low SiO₂ contents of the powders of Examples 1 to 3 is inthe pH range below 3. Since the isoelectric point is an importantparameter for the stability of dispersions, it is possible to stabilisethe powders according to the invention with small amounts of silicondioxide in the physiologically relevant pH range from ca. 5 to 7.Dispersions of titanium dioxide exhibit the lowest stability in thisrange.

The powders according to the invention of Examples 1 to 3 exhibit asignificantly lower photoactivity than pyrogenically produced P25titanium dioxide from Degussa.

The powders according to the invention from Examples 1 to 3 exhibit avery high UV absorption, which at 320 nm is greater than 97% and at 360nm is greater than 90%.

Due to the high UV absorption and the low photoactivity the powdersaccording to the invention are ideally suitable for sunscreenformulations.

The DBP absorption of the powders according to the invention of Examples1 to 3 is slight or not measurable. This indicates a low degree ofintergrowth.

Example 6 Sunscreen Agent

A sunscreen agent containing 4 wt. % of the powder according to theinvention of Example 1 was prepared according to the followingformulation (values in brackets in wt. %). Phase A: Isolan GI 34 (3.0),castor oil (1.2), Tegesoft OP (10.0), Tegesoft Liquid (5.0), glycerol86% (3.0), Phase B: Paracera W80 (1.8), isohexadecane (5.0), Phase C:powder according to the invention of Example 1 (4.0), Phase D: magnesiumsulfate (0.5), fully deionised water (66.5).

Phase A is heated in a mixer to 70° C. Phase B is melted on a magneticheating plate at 80° C. and then added to Phase A. Phase C is stirredinto the oily phase at ca. 300 rpm and under a vacuum. Phase D is alsoheated to 70° C. and added under a vacuum to the mixture of A-C.

TABLE 1 Experimental conditions in the production of the powders 1 to 5Inert Inert Vaporiser H₂ H₂ Secondary Gas Gas TiCl₄ SiCl₄ TemperatureCore Jacket Air Core Air Core Jacket Example kg/hr kg/hr ° C. Nm³/hrNm³/hr Nm³/hr Nm³/hr Nm³/hr Nm³/hr 1 3.86 0.4 140 1.45 0.9 7.7 25 0.20.5 2 3.86 0.2 135 1.45 0.9 7.7 25 0.2 0.5 3 3.86 0.11 137 1.45 0.9 7.725 0.2 0.5 4 3.86 0.81 131 1.45 0.9 8 20 0.2 0.5 5 3.86 1.15 133 1.450.9 8.3 20 0.2 0.5

TABLE 2 Physico-chemical data of the powders 1 to 5 TiO₂ SiO₂ Ex- Con-Con- Photo- Absorp- am- tent tent BET activity tion ⁽¹⁾ DBP No. ple Wt.% Wt. % m²/g Index % pH g/100 g 1 92.67 7.33 66 0.26 97.3/90.4 3.69 1212 96.19 3.8 62 0.46 97.4/92.8 3.98 n.m. ⁽²⁾ 3 97.83 2.13 57 0.4997.9/93.3 4.27 n.m. ⁽²⁾ 4 87.29 12.67 59 — — 3.75 — ⁽³⁾ 5 80.85 19.15 68— — 3.89 — ⁽¹⁾ Absorption at 320/360 nm; 3 wt. % dispersion in H₂O; ⁽²⁾n.m. = not measurable; ⁽³⁾ — = not measured

1. A powder comprising particles with a core of titanium dioxide and acoating of silicon dioxide, wherein the silicon dioxide is present in anamount of between 0.5 and 40 wt. %, the particles have a BET surface ofbetween 5 and 300 m²/g, and the particles are primary particles thathave a coating of silicon dioxide and a core of titanium dioxide.
 2. Thepowder according to claim 1, wherein the primary particles can growtogether to form aggregates.
 3. The powder according to claim 1, whereinthe silicon dioxide is present in the powder in an amount of 1 to 20 wt.%.
 4. The powder according to claim 1, wherein the titanium dioxide corehas a ratio of the rutile/anatase modifications of 1:99 to 99:1.
 5. Thepowder according to claim 1, wherein an aqueous dispersion of the powderwith a solids content of 3 wt. % has an absorption of at least 95% at320 nm and an absorption of at least 90% at 360 nm.
 6. The powderaccording to claim 1, which has a photoactivity index of less than 0.5.7. The powder according to claim 1, which has an isoelectric point at apH value of between 1 and
 4. 8. The powder according to claim 1, whereinthe BET surface is between 40 and 120 m²/g.
 9. An aggregate of particlescomprising the powder according to claim 2 and wherein the primaryparticles have grown together via the silicon dioxide coating.
 10. Asunscreen agent comprising the powder according to claim 1 in an amountof between 0.01 and 25 wt. % based on the weight of the sunscreen agent;and one or more of a UV-absorbing pigment, chemical UV filter, and asolvent.
 11. A process for the production of the powder according toclaim 1, comprising mixing a vaporisable silicon compound and avaporisable titanium compound corresponding to a subsequently desiredratio of SiO₂ and TiO₂ in the powder, vaporizing the mixture at atemperature of 200° C. or less transferring the vaporized mixture in aninert gas stream together with hydrogen and air or with oxygen-enrichedair into a central pipe of a burner forming a reaction mixture, ignitingthe reaction mixture at mouth of the burner in the presence ofadditional, secondary air, combusting the reaction mixture in a cooledflame pipe generating gaseous reaction products, removing the titaniumdioxide powder coated with silicon dioxide from the gaseous reactionproducts, wherein the ratio of air to secondary air is greater than 0.3,hydrogen to secondary air is greater than 1, vaporisable titaniumdioxide compound to secondary air is greater than 0.5.
 12. The processaccording to claim 11, wherein titanium tetrachloride is the titaniumcompound.
 13. The process according to claim 11, wherein silicontetrachloride is the silicon compound.
 14. The process according toclaim 11, further comprising freeing the gaseous reaction product fromadhering hydrogen chloride following the removal of the titanium dioxidepowder coated with silicon dioxide from the gaseous reaction products.15. A method of making a dispersion, comprising mixing the powderaccording to claim 1 with a solvent.